Usually, traditional oil and gas pipelines are made by Nb alloying and controlled rolling, which results in that the yield-tensile ratio of pipeline steel is relatively high, normally, greater than or equal to 0.85, thus, this type of pipeline steel is not adapted to manufacture transportation pipelines used in high-activity seismic areas.
CN101962733A discloses an X80 high-deformability pipeline steel with low cost and high toughness and the manufacturing method thereof, wherein C: 0.02-0.08%, Si≦0.40%, Mn: 1.2-2.0%. P≦0.015%, S≦0.004%, Cu≦0.40%, Ni≦0.30%, Mo: 0.10-0.30%, Nb: 0.03-0.08%, Ti: 0.005-0.03%, and the technology thereof is adopted that the soaking temperature is 1200-1250° C., the rolling finishing temperature of the recrystallization zone is 1000-1050° C., the rolling starting temperature for finish rolling is 880-950° C., and the rolling finishing temperature thereof is 780-850° C.; the steel is air-cooled by two stages at speed of 1-3° C./s to the temperate which is 20-80° C. below Ar3, thereby obtaining 20-40% ferrite; laminar cooled at speed of 15-30° C./s to 250-450° C., obtaining steel plate with ferrite (20-40%)+bainite+martensite (1-3%) whose yield strength is 530-630 MPa, tensile strength is 660-800 MPa, uEL is ≧10%, and the yield-tensile ratio is ≦0.80. The properties such as yield-tensile ratio and elongation of the steel plate cannot yet meet the requirements on resistance to large deformation of the transportation pipelines used in high-activity seismic areas.
Therefore, currently a steel plate with low yield-tensile ratio and high toughness is needed for manufacturing transportation pipelines used in high-activity seismic areas which can resist large deformation.